Sparkplugs for internal combustion engines such as those disclosed in German Patent Disclosure document No. 24 04 454 and German Patent No. 1 208 120 have a metallic cylindrical or tubular housing which has a ground electrode in its ignition end. The metallic housing envelops an insulator which has a lengthwise bore. The ignition end of the sparkplugs contains an electrical connecting pin extended inwardly in the bore of the insulating portion of the sparkplug. The known sparkplugs have the disadvantage that because different materials are used for the insulator and for the central electrode (connecting pin) stresses may be set up between them, particularly under operating conditions which involve high temperatures which can lead to damage to the insulator.
British Patent No. 198 345 discloses sparkplugs having a central electrode which is made of a non-metallic electrically conductive material having a coefficient of thermal expansion which corresponds substantially to the coefficient of thermal expansion of the insulator. This avoids the formation of stresses between the insulator and the central electrode. However because the insulator and the non-metallic central electrode are composed of different materials, undesireable sinter reactions can occur at the contact between the insulator and the central electrode which reduce the functional reliability of the sparkplugs and sometimes also reduce their service life.
It is an object of the present invention to provide an improved sparkplug which avoids the disadvantages of the art and provides more uniform and reliable operation during its service life and which has a long service life.